The present invention relates generally to aircraft and, more particularly, to a spanwise transition section having a negative sweep angle which interconnects the body and the wing of a blended wing-body aircraft.
There are primarily two types of aircraft configurations: the more common (conventional) configuration which includes a tail section comprised of vertical and horizontal stabilizers located at the aft end of a tubular fuselage; and the tailless configuration. As to the latter, there are two sub-types: a first type which has no central body, commonly known as a "flying wing," and a second type having a central body which is blended into laterally extending wings, commonly referred to as a "blended wing-body" aircraft.
A generic example of a conventional aircraft having a tail section is schematically shown in FIG. 1, and designated as aircraft 20. Aircraft 20 includes tubular fuselage 21, wing 23, horizontal stabilizer 25, and vertical stabilizer 27. When loaded, aircraft 20 has center of gravity 29. Horizontal stabilizer 25 controls the rotation of aircraft 20 about the pitch axis passing through center of gravity 29. Vertical stabilizer 27 controls the rotation of aircraft 20 about the vertical, or "yaw," axis passing through center of gravity 29.
The vector L represents the lift generated by wing 23. The additional lift generated by fuselage 21 is small in comparison to the lift generated by wing 23, and will be ignored for the limited purpose of this brief discussion. The vector l represents the lift generated by horizontal stabilizer 25. L acts in the upward, or positive direction, while l acts in the opposite, or negative direction. L has a magnitude much larger that of l. The angle of attack of aircraft 20 is controlled and stabilized by the pitch moments about center of gravity 29 generated by L and l.
The necessary presence of horizontal stabilizer 25 and vertical stabilizer 27 causes a significant increase in the drag coefficient for aircraft 20 in comparison to what the drag coefficient would be in the absence of the two aforementioned control elements. Another drawback inherent to aircraft 20 is the weight of fuselage 21, which serves to provide a pitch moment arm of sufficient length to allow the pitch rotation of aircraft 20 to be controlled by the lift l generated by horizontal stabilizer 25, as well as to provide a yaw moment arm of sufficient length to allow the yaw rotation of aircraft 20 to be controlled by the force vector generated by vertical stabilizer 27.
Also, in order to sustain flight, L must have a magnitude sufficient to lift both wing 23 and fuselage 21. L must thus exceed the weight of wing 23. As a consequence, wing 23 will be subjected to a resultant upward force equal to L minus the weight of wing 23. This resultant force subjects wing 23 to a bending moment, with the maximum moment occurring at the wing root where wing 23 joins fuselage 21.
Wing 23 must be designed to withstand this bending moment induced by the resultant force, in addition to the dynamic forces and moments created by aircraft maneuvers. More particularly, wing 23 must be designed stronger than would be the case in the absence of the aforementioned resultant force, and this strengthening requires more structural weight than would otherwise be required. The strengthening of wing 23 also typically takes up additional volume that might otherwise by utilized to carry fuel. Both of the foregoing factors reduce the range of aircraft 20.
The foregoing drawbacks inherent to conventional aircraft designs exemplified by aircraft 20 have led aeronautical engineers to consider tailless designs. A perspective view of tailless aircraft 30, a generic example of a tailless aircraft, is shown in FIG. 2. Aircraft 30 includes main wing section 31, deflectable reflexes 33, deflectable flaps 35, wing tip 37, and center of gravity 39.
FIG. 3 provides a side view of wing tip 37 and depicts reflex 33 in greater detail. Main wing section 31 generates upward, or positive, lift vector L, whereas each reflex 33 generates a lift vector l acting in the opposite, or negative, direction. The flight of tailless aircraft 30 is controlled and stabilized by the appropriate deflections of flaps 35 and reflexes 33.
As may be discerned by cursory inspection of FIG. 3, tailless aircraft 30 has no horizontal and vertical stabilizers projecting into the ambient airstream, and thus has a lower drag coefficient than aircraft 20. Moreover, since the flight of aircraft 30 is controlled and stabilized without horizontal and vertical stabilizers, it does not require the moment arm to the aforementioned stabilizers otherwise provided by a fuselage. The absence of a fuselage further lowers the drag coefficient and weight of tailless aircraft 30 in comparison to aircraft 20. Wing section 31 also realizes a savings in weight compared to wing 23 of aircraft 20 because it need not be designed to withstand the moment generated by having to lift a fuselage in addition to its own weight.
Though offering the aforementioned advantages over aircraft having fuselages and tail sections, tailless aircraft suffer from several inherent design problems. To begin with, with the tailless aircraft 30, the pitch moment arm from center of gravity 39 to the negative lift l generated by reflexes 33 is shorter than the corresponding pitch moment arm for conventional aircraft 20 between center of gravity 29 and the negative lift l generated by horizontal stabilizer 25. This renders aircraft 30 more sensitive to changes in the axial station of center of gravity 39, for example, due to a shift in the location of cargo or fuel during flight, or the placement of cargo during loading on the ground.
Alternatively stated, the aerodynamic envelope for stable and controlled flight for tailless aircraft 30 is narrower and thus will tolerate less movement of loaded center of gravity 39, in comparison to the wider envelope for conventional aircraft 20. This characteristic makes it more challenging to design a commercial airliner using a tailless aircraft because it is difficult to consistently and accurately predict the load and to control the seating location of the passengers on a commercial passenger flight, in comparison to a flight carrying only cargo, weapons or military personnel.
Tailless aircraft share a further shortcoming that arises from the commercial realities facing airlines and the designers and builders of commercial airliners. More particularly, modem commercial airliners are typically designed and built as one model in a family of derivative configurations. For conventional aircraft exemplified by aircraft 20, each model varies primarily in the length of its tubular fuselage, with the various family members sharing a similar wing and avionics. By using different members of a manufacturer's family of airliners, the airline company's pilots, mechanics, and other support personnel need only acquire detailed knowledge of one model in the family. They are subsequently able to fly, maintain and repair another model in the same family with substantially less instruction and training than would be required to acquire proficiency with a completely new and unfamiliar aircraft.
The primary means of creating a new model from an existing aircraft is by inserting a hollow axial plug having the identical diameter of the original fuselage, into the fuselage. This increases the size of the original aircraft and avoids the significant investment necessary to develop a completely new model. An airline company will select a model based on the predicted passenger load and the length of the route the aircraft is to service.
Since a tailless aircraft obviously does not have a fuselage whose length can be readily changed, this design does not lend itself to such a relatively straightforward modification which would allow a manufacturer to inexpensively modify a tailless base model and develop a family of airliners to satisfy the market driven requirements of the airline companies. The foregoing characteristic inherent to the configuration of tailless aircraft has impeded the commercial development of a tailless aircraft in spite of its considerable aerodynamic efficiency.
Based on the foregoing, it can be appreciated that there presently exists a need for a tailless aircraft which overcomes the above described disadvantages and shortcomings of the tailless aircraft of the prior art. The present invention facilitates the design of such an aircraft, and thereby fulfills this need in the art.